A radio communication device of a mobile communication system, for example, uses an amplifier to amplify a signal of high frequency (see, e.g., Japanese Laid-Open Patent Publication No. 2010-268248, International Publication No. 2009/131138, Published Japanese-Translation of PCT Application, Publication No. 2002-510927, and Japanese Laid-Open Patent Publication No. 2009-213090). In the amplifier, for example, an impedance matching circuit is disposed on the input side and the output side of an amplifying element.
The impedance matching circuit on the input side of the amplifier, for example, converts the characteristic impedance (e.g., 50[Ω]) of an input terminal of the amplifier to a complex conjugation of the input impedance of the amplifying element. The impedance matching circuit on the output side of the amplifier, for example, converts the characteristic impedance (e.g., 50[Ω]) of an output terminal of the amplifier to the complex conjugation of the output impedance of the amplifying element.
To design an electronic amplifier of high efficiency, the impedance matching circuit on the output side is designed so that, for example, the impedance of the output terminal as seen from the amplifying element will be converted to the point of load of the highest efficiency. To design an electronic amplifier of high gain, the impedance matching circuit on the input side is designed so that, for example, the impedance of the input terminal as seen from the amplifying element will be converted to the point of load of the highest gain.
In the conventional technologies described above, however, since the impedance matching circuit has impedance frequency characteristic, when a wideband signal is input, the impedance within the band of the signal can become uneven.